The present invention relates generally to the field of magnetic data storage and retrieval systems. More particularly, the present invention relates to a transducing head having a magnetoresistive sensor stabilized by permanent magnet bias elements having a low magnetic moment and a high coercivity.
A transducing head of a magnetic data storage and retrieval system typically includes a magnetoresistive (MR) reader portion for retrieving magnetic data stored on a magnetic media. The reader is typically formed of several layers which include an MR sensor positioned between two insulating layers, which are in turn positioned between two shield layers. The MR sensor may be any one of a plurality of MR-type sensors, including, but not limited to, anisotropic magnetoresistive (AMR), giant magnetoresistive (GMR), tunneling giant magnetoresistive (TMR), spin valve, and spin tunneling sensors.
When the transducing head is placed near a magnetic medium, a resistance of the MR sensor fluctuates in response to a magnetic field emanating from written transitions in the magnetic medium. By providing a sense current through the MR sensor, the resistance of the sensor can be measured and used by external circuitry to decipher the information stored on the magnetic medium.
To operate the MR sensor properly, the sensor must be stabilized against the formation of edge domains because domain wall motion results in electrical noise that makes data recovery difficult. A common way to achieve stabilization is with a permanent magnet abutted junction design in which permanent magnet bias elements directly abut opposite sides of the MR sensor. Permanent magnets have a high coercive field (i.e. are hard magnets). The magnetostatic field from the permanent magnets stabilizes the MR sensor, prevents edge domain formation, and provides proper bias.
In recent years, MR sensor widths have been decreased to accommodate ever-increasing areal densities of magnetic media. But, with a decrease in MR sensor widths, it has been important to maintain constant MR sensor output by increasing MR sensor sensitivity. In prior art designs, this goal has been accomplished by several methods, including decreasing a thickness of a sensing layer of the MR sensor and/or reducing a thickness of the permanent magnet bias elements and/or recessing the permanent magnet bias elements a distance from the MR sensor (a method introduced by U.S. patent application Ser. No. 10/027,051, hereby incorporated by reference) and/or shortening a length of the permanent magnet bias elements (a method introduced by U.S. patent application Ser. No. 10/348,386, hereby incorporated by reference).
In the case of reducing the permanent magnet thickness, process-control issues exist with creating ever-thinner permanent magnet layers in a volume manufacturing environment. Namely, it is difficult with thinner permanent magnets to achieve consistent thicknesses of the layers, particularly across a wafer upon which tens of thousands of MR sensors are built. That is, the permanent magnets formed near the center of the wafer may be thicker than the permanent magnets formed near the edge of the wafer. Also, the photolithographic processes employed in forming the permanent magnet layer may result in the two permanent magnets associated with one MR sensor having unequal thicknesses. As the thickness of the permanent magnet bias elements is decreased, this asymmetry in thickness becomes a substantially large percentage of the total MR sensor thickness. For instance, an asymmetry of 50 Angstroms would result in a 50% difference in thickness across the wafer for a targeted 100 Angstroms thick permanent magnet, whereas it would be only a 10% difference for a targeted 500 Angstroms thick permanent magnet.
In addition to permanent magnet asymmetry, error may arise in the permanent magnet positioning with respect to a sensing layer of the MR sensor. The positioning error may result from a variety of factors, including thickness variation of deposited layers in the process of forming a MR sensor and photolithography process variations in the process of forming a MR sensor. The positioning error may result in a response variation of a MR sensor.
Thus, there is a need for a MR sensor design having increased sensitivity without requiring a decrease in thickness of the permanent magnets.